1. Field of the Invention
This invention relates to a diffracting optical element, and an optical system, a photographing apparatus and an observation apparatus having the diffracting optical element, and particularly to a diffracting optical element for use in a wavelength area relatively having a band width such as a visible light area.
2. Related Background Art
As the features of a diffracting optical element, mention may be made of the following:
(1) In contrast with a refracting optical system having power of the same sign, the way in which chromatic aberration reveals itself is converse.
(2) By changing the pitch of the relief pattern of a diffraction grating constituting the diffracting optical element, a wave front can be controlled.
(3) The relief pattern is of very thin structure and is therefore small in occupied volume.
Particularly regarding item (1) above, if the diffracting optical element is introduced into what is generally comprised of a refracting optical system such as a camera lens, chromatic aberration will be greatly improved.
Further, by item (2) above, the effect of introducing a so-called aspherical lens into an optical system can also be obtained.
Furthermore, by adding item (3) above, there can be realized a compact optical system having good optical performance.
Such techniques are disclosed in literature such as SPIE Vol. 1354, International Lens Design Conference (1990), Japanese Patent Application Laid-Open No. 4-213421 (corresponding U.S. Pat. No. 5,044,706), Japanese Patent Application Laid-Open No. 6-324262 (corresponding U.S. Pat. No. 5,790,321), etc.
FIG. 1 of the accompanying drawings shows the epitome of the conventional most popular diffracting optical element. As shown in FIG. 1, a relief pattern 101 is formed on the boundary between an air layer 102 and a resin material layer 103 having a refractive index Nd=1.497 and an Abbe number νd=57.44. The height of this relief pattern 101 is represented by h, and the pitch thereof is represented by P. Such a diffracting optical element comprised of a diffraction grating will hereinafter be called a single-layer diffracting optical element.
When P=150 μm and h=1.05 μm, the result of the calculation of the diffraction efficiency of this diffracting optical element 105 is shown in FIG. 2 of the accompanying drawings. In FIG. 2, the axis of abscissas represents the wavelength 400 nm–700 nm of a visible light area, and the axis of ordinates represents the diffraction efficiency of +first-order diffracted light. This diffracting optical element is such that in the used wavelength area 400 nm–700 nm, +first-order is chosen as the design order at which the diffraction efficiency of diffracted light becomes highest. It is also possible to set so as to change the height of the relief of the diffracting optical element, whereby the diffraction efficiency of other order than +first-order may become highest, but there will be shown hereafter a case where +first-order is chosen as the design order and the diffraction efficiency of +first-order diffracted light becomes highest.
According to FIG. 2, in the visible area, diffraction efficiency lowers greatly in the short wavelength side and long wavelength side wavelength areas. In these wavelength areas, the diffraction efficiency of other unnecessary orders (not shown) than +first-order becomes high. When such a diffracting optical element is applied to an optical system such as a camera lens used in the visible light area, the unnecessary orders may cause flare.
When the height of the diffracting optical element is defined as h and the refractive index thereof in a certain wavelength λ is defined n(λ), the optical path difference OPD occurring between it and the air (refractive index 1) isOPD=(n(λ)−1)·h. The diffraction efficiency at this time is   η  =      [                            sin          ⁡                      (                          π              ·              x                        )                          2                    π        ·        x              ]  wherex=(OPD/λ)−m and now the design order is +first-order and therefore m=1. The diffraction efficiency η becomes highest whenx=0
FIG. 3 of the accompanying drawings shows the value (phase characteristic) of x of the diffracting optical element in FIG. 1 (m=1). The value of x deviates greatly from 0 on the short wavelength side and the long wavelength side and therefore assumes the characteristic as shown in FIG. 2.
The technique of eliminating the wavelength dependency of the diffraction efficiency of such a diffracting optical element, and preventing the occurrence of flare or the like is disclosed in applicant's (or assignee's) Japanese Patent Application Laid-Open No. 11-223717 (corresponding U.S. Application Laid-Open No. 2001015848), or in Japanese Patent Application Laid-Open No. 9-325203 (corresponding U.S. Pat. No. 6,157,488) or Japanese Patent Application Laid-Open No. 9-127322 (corresponding U.S. Pat. No. 6,157,488). These aim to construct a diffracting optical element by combining two or more kinds of materials differing in optical characteristic, and reduce the wavelength dependency of diffraction efficiency.
FIG. 4 of the accompanying drawings shows an example of the construction of a diffracting optical element described in an embodiment in Japanese Patent Application Laid-Open No. 11-223717. A laminated type diffracting optical element will hereinafter be described by the use of this example of the construction. In FIG. 4, a laminated type diffracting optical element 111 is formed by a relief pattern 106 constructed between an optical material layer 109 in which Nd=1.635 and νd=22.99 and an air layer 108, and a relief pattern 107 constructed between an optical material layer 110 in which Nd=1.5129 and νd=51.00 and the air layer 108. The heights h1 and h2 of the relief pattern 106 and the relief pattern 107, respectively, are h1=−7.88 μm and h2=10.95 μm. The reason why h1 is given the minus sign is that the direction of grating structure forming the diffracting optical element is opposite to that of h2.
In this example of the construction, the relation between the aforementioned x and the wavelength is shown in FIG. 5 of the accompanying drawings. Also, FIG. 6 of the accompanying drawings shows the wavelength dependency of the diffraction efficiency of the first-order light of the laminated type diffracting optical element shown in FIG. 4, and in FIG. 6, as compared with FIG. 2, the diffraction efficiency of the short wavelength side and the long wavelength side is greatly improved. However, it will be seen that in the short wavelength side area, there is a reduction in diffraction efficiency.
FIG. 7 of the accompanying drawings shows the refractive indices of two optical materials forming the diffracting optical element in FIG. 4. In FIG. 7, dotted line and solid line represent the refractive indices, respectively, of the materials forming layers 109 and 110. A change in the refractive index of the material forming the layer 109 on the short wavelength side thereof is great and therefore, the change in x on the short wavelength side in FIG. 5 is great. This is the cause of the reduction in the diffraction efficiency on the short wavelength side.
As previously described, as compared with the single-layer type diffracting optical element, the diffraction efficiency of this laminated type diffracting optical element is greatly improved, but there is still a reduction in diffraction efficiency on the short wavelength side. This may cause flare and therefore, the achievement of a diffracting optical element which is high in diffraction efficiency in the whole of a wavelength area used is desired.